In high performance electronic components such as BGA's (ball grid array packages) and CSP's (chip size packages), a semiconductor element and a substrate are bonded to each other by die bonding using a bonding material. Die bonding refers to a process in which a semiconductor element obtained by cutting of a silicon wafer is secured to a support substrate therefor. Die bonding of a semiconductor element and a substrate is carried out so that when the semiconductor element generates heat during use of electronic equipment, the semiconductor itself is prevented from undergoing a decrease in performance or thermal damage under the influence of heat. Namely, as a result of die bonding of a semiconductor element and a substrate, the semiconductor element and the substrate are intimately contacted with each other, and heat generated from the semiconductor element is dissipated through the substrate. Die bonding is also carried out in order to electrically ground a semiconductor element.
Adhesive resins and solder are generally used as materials for die bonding. If the objective of die bonding is dissipation of heat from a semiconductor element, an adhesive resin is satisfactory, but when heat dissipation and electrical grounding are the objectives, it is preferable to use solder, which has good thermal conductivity and good electrical conductivity.
In die bonding of a semiconductor element and a substrate, when a solder preform is placed on a substrate and heated so that the solder preform melts, the resulting molten solder is pressed out from between portions to be soldered under the weight of the semiconductor element, and the amount of solder between portions to be soldered sometimes becomes small. Bonding by means of solder requires a suitable amount of solder between portions to be soldered in order to exhibit the inherent bonding strength of solder. However, if as described above solder between portions to be soldered is forced out by the weight of a semiconductor element placed atop the solder, the clearance between the semiconductor element and the substrate becomes too small, and an adequate amount of solder cannot be present, resulting in a decrease in bonding strength. Namely, a maximum bonding strength can be exhibited in soldering if the clearance is suitable, but the bonding strength of the joint as a whole becomes weak if portions having a small clearance are locally formed.
In order to perform soldering such that portions to be soldered are suitably separated and a suitable amount of solder is maintained between the portions to be soldered, a plurality of high melting point metal particles having a melting point higher than solder (referred to below simply as metal particles) such as metal particles of Ni, Cu, Ag, Fe, Mo, or W have been interposed between the portions to be soldered. However, placing metal particles between portions to be soldered each time soldering is carried out is extremely troublesome and results in poor production efficiency, so solder preforms having metal particles previously interspersed therein have been used.
Patent Document 1 discloses a roller embedding method in which metal particles are dispersed atop a solder sheet using a hopper, and the metal particles are embedded in the solder sheet using an embedding roller. As the metal particles, round, triangular, cylindrical, or plate-shaped particles, minute particles, or the like are used.
Patent Document 2 also discloses a roller embedding method. The resulting solder preform has metal particles with a particle diameter of 30-70 micrometers embedded in solder.
Patent Document 3 discloses a method of producing a composite solder ingot having metal particles dispersed therein. In that method, a mixture of metal particles and a flux which have been kneaded together is charged into molten solder and stirred, cooled, and solidified. The particle diameter of the metal particles used in this case is 20 micrometers, 50 micrometers, or 100 micrometers.
Patent Document 4 discloses a method of producing a solder preform in which superimposed sheets having metal particles dispersed from a hopper between two solder sheets are subjected to rolling such that the two sheets are adhered to each other. That method uses metal particles having an average particle diameter of 30-300 micrometers, with the standard deviation of the particle diameter being at most 2.0 micrometers.
Patent Document 1: JP H03-281088 A
Patent Document 2: JP H06-685 A
Patent Document 3: JP H06-31486 A
Patent Document 4: JP 2005-161338 A